To grow a crystal by vapour phase crystal growth techniques, it is necessary to provide a source of the materials from which the crystal is to be formed, and a seed onto which the crystal is grown from the source material. A transport zone links the source and sink zones. By creating a temperature difference between the source zone and the sink zone where the seed is provided, a vapour pressure difference will be created between the source and sink zones which acts as the driving force for crystal growth. The temperature of the source zone should be greater than the temperature of the sink zone.
The control of the crystal growth will be dependent on a number of factors, including the source and sink temperatures and the vapour pressures.
In early vapour phase crystal growth systems, the crystal was grown on a seed crystal provided in a sealed, tubular, quartz ampoule from source material also provided in the ampoule. The ampoule was provided in a tubular furnace to heat the source and sink zones. However, it is very difficult to control the growth of crystals using such a system.
The provision of a first flow controller between the source and seed zones, in combination with a second flow controller downstream of the seed zone to provide continuous pumping to remove a proportion of the source material from the seed zone was suggested to improve the basic tubular system. The provision of the first flow controller acts to make the mass transport rate less sensitive to the temperature differential between the source and sink zones than in the basic tubular system, although the temperature difference is still an important factor.
An alternative apparatus is disclosed in WO 99/10571 in which independent heating means are provided for each of the source and sink zones. The source and sink zones, together with the independent heating means, are provided in a vacuum chamber. This enables more accurate control of the source and sink temperatures, and therefore of the temperature difference between these zones, to enable a solid-vapour-solid phase transition in the source, transport and sink zones. However, the heating means must be one able to operate in a vacuum. Therefore, heaters such as resistance coil heaters are unsuitable. A flow restrictor can be provided in the transport zone between the source and sink zones to provide additional control.